Terminal is actually going into power saving mode.

The wireless terminals will go into this mode after 10-20 minutes of inactivity to reserve battery life.   If the merchant presses the OK button or attempts to use the terminal, the screen will turn on.  Please make sure to convey this to the merchants.  The terminal is not defective or need to be replaced. 

Changing Power Down Timer: In Utility>Settings>Power Management>Saving Timer>Shutdown timer> set for 999.

If a terminal is in sleep mode, tapping the power button should reactivate it.